I personally have a two horse slant (horses like it). In your situation I think you might be happier with a stock type trailer with divider to give the horses more room (as one of the other posters mentioned). I don't think anyone has mentioned this yet but check out the suspension system on the trailer you're thinking of getting. I went from a leaf spring suspension in my old trailer to a rubber torsion suspension with my new trailer and it makes a world of difference for a smooth ride.
As an aside to the trailer issue, I'd just like to add secure a list of emergency phone numbers of various places along the way (vets, trailer repairs, etc). Also include an emergency medical kit, extra halters/lead ropes, etc.